understanding fork in c

Question

#include <stdio.h>
int num = 0;
int main(int argc, char*argv[]){
   printf("common line\n");  
   printf("%d", num); 
   int pid;
   pid = fork();
   if(pid == 0){       /*child*/
      num = 1;
   } else if(pid > 0){  /*parent*/
      num = 2;
   }
   printf("%d\n", num);
}

I above program, common line string is shown single time in output. But "0" is comming twice in the results.

Coming outputs:

common line
01
02

OR

common line
02
01

As per my understanding, 0 should come only once?

possible duplicate of [About fork and printf/write](http://stackoverflow.com/questions/14647468/about-fork-and-printf-write) — R Sahu, Mar 10 '15 at 18:54
@MartinJames that solved my problem... but what it suggests? i am little confused with this behaviour. When i am not flushing, then also 0 should come once in the buffer — Vishwadeep Singh, Mar 10 '15 at 18:59
@MartinJames even if i add new line to first printf, that also colves my problem — Vishwadeep Singh, Mar 10 '15 at 19:03
When you fork, all data is duplicated in the child process. If you fork without flushing stdout, buffered data (the 0 here) is duplicated in the child process: a '0' is printed by both process when stdout is flushed AFTER forking. — Joël Hecht, Mar 10 '15 at 19:12

score 3 · Accepted Answer · answered Mar 10 '15 at 19:10

By adding a newline character at the end of your string you are implicitly flushing the output buffer before you fork. Your other option is to explicitly flush it with fflush(stdout). Otherwise when you fork(), both processes are just spitting out whatever was still left in the buffer beforehand (in your case the unflushed buffer still contains num from printf("%d", num)).

understanding fork in c

1 Answers1